It's fair to say nine out of 10 cars born in the '60s and '70s now have some sort of electrical problem. It may drain the battery in a couple of days, or have a cigarette lighter or dome light that keeps blowing a fuse. Or maybe it's a turn signal or brake light that fails to illuminate. Sometimes people hack into the factory circuit to juice some aftermarket accessory, causing all sorts of problems. Early fuse boxes don't have much room to adapt to the ever changing and complicating automotive movements.

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Today we're adding things like electric fuel pumps, electronic speed sensors, digital ignition controls, and upgrading cars with power accessories that didn't come with them. All these power consumers need to be fed, and the only way to do that is to wire up a legitimate power source with a fuse or relay to distribute power as necessary.

3/21This is American Autowire's Classic Update kit for '64-66 Mustang, PN 510125, that retails for $659. It comes with absolutely everything you will need to rewire a car. No need to order any special parts, switches, connectors, or hardware. You get all of the dash switches, connectors, and wire to make every component-factory or not-light up with power.

If you've got more than one add-on planned, it can get messy real quick. The Project Street Fighter Mustang was a bare-bones optioned car with barely enough wires to run the straight-six and one headlight. Through its life, the 'Stang had been subject to questionable wiring repairs. With these violations to the factory wiring, there was no way it could handle the load of an electric fuel pump, electronic ignition, upgraded headlights, aftermarket electronic gauges, or anything else we may have in store for it. The ultimate solution is to replace the entire harness with something that has our future updates accounted for.

For over 20 years, American Autowire has been a huge player in the GM vehicle harness game. They got their start making a harness for first-gen Camaros so they could have a way to swap in gauges easily. It was 2008 when they debuted their first Classic Update Mustang harness to fit '67-68 Mustangs in Chad McQueen's Bullitt Mustang at the SEMA convention. Since then, they've been adding more Mustang harnesses to their catalog, including one that covers '64-66 Mustangs. The Classic Update kit is intended for restoration projects as well as highly modified cars. The heart of the harness is the fuse box packed with modern, flat-blade fuses and sometimes double the circuits found in the original box. From there the wires are grouped much like the factory to route in similar locations to mate up to stock components. American Autowire takes pride in the fact that they supply all of the necessary connectors to rewire your car from head to toe with nothing extra to buy for $659 (PN 510125). The kit includes correct headlight and taillight sockets, ignition switch, headlight switch, high-beam switch, along with the harnesses for any special factory components.

The Pro Touring movement has put a huge demand on vintage tin to feature goods they were never intended to have. Late-model engines and suspensions with far more electronic components such as electric senders for coolant temp, oil pressure, speed sensors, to name a few, have been finding their way between the fenders of early muscle cars. The Classic Update harness provides wiring and circuits to power them.

4/21Included in the kit is a template to mount the new fuse box. It uses the firewall's factory impressions to guide you to its ideal mounting position. Both a template for the interior and engine side of the firewall is included so you can use what you have better access to.

Our Mustang isn't ready to run, but it needed a harness if we were going to get any closer. While the interior was still gutted for the 'cage project, we opted to lay the harness in now, and wire up the parts as we get them. Coming soon are gauges, alternator, ignition, and starter solenoid that will be easily stitched in with the wires provided in the kit.

Tools You'll Need
Heat-Shrink Tubing and a Heat Gun
Heat-shrink tubing is great when you're working with non-insolated connectors or a wire loom that frays. You can get a heat gun from Harbor Freight for $10. If that price is too steep, using a lighter works, you just need to be careful not to burn anything. Heat-shrink tubing is also available at Harbor Freight in an assortment of sizes for $8. Both of these items are things you should keep around for future projects.

Electrical Tools
To cut, strip, and crimp a wire, you can get away with using one multi-tool, but serious wiremen usually find specific tools they become accustomed to. The orange-handled tool shown above is a fancy wire stripper that holds the wire with one set of jaws, and pulls the insulation away with the other. The large red-handled tool is an MSD ratcheting crimper that has removable jaws for standard connectors, Weatherpack connectors, and spark plug wire ends. The smaller red-handled tool is a traditional crimper that's better for hard-to-reach areas. Keep standard electrical connectors around just in case you need to modify something, though American Autowire includes all of the connectors you should need for the kit.

5/21Taking a closer look at the fuse box, you can see the glass-tube fuses have been swapped for modern flat-blade fuses. Also, a couple new circuits have been added, many of which we won't need for the Mustang, but it's nice to have them built-in, in case our plans change down the line.

Zip Ties/Side-Cutting Pliers
When it comes to zip ties, it pays to buy in bulk! A 1,000-count, 4-inch zip-tie back was $11 from the local electrical supply store. Side-cutting pliers are the best tool for cutting the tails off the zip ties aside from a zip-tie gun.

Braided Loom
Wire sheathing is the key to cleaning up exposed wiring. We used this nylon material that expands as you push it together, like a Chinese finger trap. It grows to about double its taut dimension to allow you to easily push it along a wire and over connectors. On the small side there is sleeving meant to hold a wire as small as 3/32 inch that expands to about 1/4 inch. At the large side of the scale is the sleeving that at its smallest diameter is 1.5 inches but can hold up to a 3.5-inch bundle. There are different varieties of sleeving to accommodate heavy-duty or high-temperature conditions as needed. The two sizes most commonly used in the Mustang's harness are the 1/4- and 3/8-inch spools. They are very reasonably priced, so keeping this stuff in stock is a great idea.